%23
is_concluded(contract(A,B,C),T):-
	become_effective(acceptance(B,A, offer(A,B,C)),T).
%15(1)
become_effective(offer(A,B,C),T1):-
	reach(offer(A,B,C),B,T1).
%18(2)
become_effective(acceptance(B,A,C),T):-
	reach(acceptance(B,A,C),A,T).
%case3b
reach(offer(anzai,bernard,sale(agriculture_machine,'$50000')),bernard,t0409).
reach(acceptance(bernard,anzai,offer(anzai,bernard,sale(agriculture_machine,'$50000'))),anzai,t0409).